Sodium reacts highly with water and emits H_2 . The reaction is highly exothermic and you observe rise in temperature. The emitted H_2 will get burnt due to this heat release and burns with a bright flame. Hydrogen can sometimes give away tiny 'pop' sound of explosions.

During the reaction, NaOH is formed and it makes the solution alkaline. Since NaOH is a very strong base, it will make the univaersal indicator turn purple.